The study, led by Riley Barton of the Marine and Coastal Research Laboratory at Pacific Northwest National Laboratory and Rensselaer Polytechnic Institute, together with Sasha Wagner, Christina M. Richardson, Alex Collins, Margaret A. Zimmer, Maya Montalvo, and Allison N. Myers-Pigg, focused on five small coastal mountain watersheds in California that were burned by the CZU Lightning Complex Fires of 2020. That fire event, ignited by an extraordinary lightning storm, tore through the Santa Cruz Mountains and left steep coastal drainages draped in ash and charred vegetation. Because these watersheds drain directly toward the Pacific, they offered an ideal natural laboratory for tracking how fire-altered carbon moves from land to sea.

The researchers zeroed in on a molecule called levoglucosan, an anhydrosugar that forms when cellulose and hemicellulose, the structural carbohydrates of plant tissue, are burned at moderate temperatures. Levoglucosan has long served as a molecular fingerprint for biomass burning in atmospheric science, because it is produced in abundance during combustion and can travel long distances in smoke. But in aquatic systems, levoglucosan has a very different personality: it is highly soluble, which means it dissolves readily in the first rains after a fire, and it is biolabile, meaning microbes can consume it quickly as a carbon and energy source. In other words, levoglucosan is the ephemeral, fast-cycling counterpart to the stubborn, centuries-persistent black carbon that has dominated discussions of fire-derived carbon in rivers.

Black carbon, by contrast, is the charred, condensed aromatic material that gives burned soils their dark color. It is famously recalcitrant, resisting microbial decay and persisting in soils, sediments, and the open ocean for hundreds to thousands of years. Because of this stability, black carbon has been treated as the primary long-term sink in the fire-carbon equation, and much of the aquatic research on pyrogenic carbon, often abbreviated PyC, has centered on quantifying these durable fractions. The new study asked a deceptively simple question: if you measure both the labile and the recalcitrant fire-derived carbon in the same streams at the same time, do they behave the same way?

The answer, emphatically, is no. Levoglucosan concentrations in the streams peaked during the earliest post-fire flows, the very first storm-driven runoff events after the flames were out, and then declined steadily over time. Remarkably, this decline was independent of stream discharge. It did not matter how much water was coursing through the channels; the levoglucosan signal faded because the supply was being exhausted, flushed out of the burned landscape during those initial pulses and consumed by microbes along the way. Black carbon told the opposite story. Its concentrations were discharge-driven, rising when flows rose, and, counterintuitively, they increased with time since the fire. The researchers attribute this delayed pattern to gradual solubilization: the charred material left on the landscape does not dissolve immediately but slowly becomes available to water over months and years as physical and chemical weathering break it down.

This divergence matters because it means that the two major categories of pyrogenic carbon operate on fundamentally different clocks. Labile anhydrosugars are a flash flood of bioavailable carbon, delivered within the first post-fire storm seasons and efficiently transported all the way to coastal lagoons and nearshore zones. Black carbon is a slow-release reservoir, accumulating in export pathways over longer timescales. A monitoring program that samples a burned watershed a year or two after a fire, or one that focuses only on the recalcitrant fraction, would completely miss the early, intense pulse of labile carbon, and with it, a significant chapter in the post-fire carbon budget.

The downstream consequences could be substantial. When levoglucosan and other soluble, easily degraded combustion products arrive in coastal lagoons and nearshore waters, they provide a fresh fuel source for microbial communities. Enhanced microbial processing means enhanced remineralization, the conversion of organic carbon back into carbon dioxide, potentially altering the carbon balance of these transitional ecosystems and influencing oxygen demand, nutrient cycling, and food web dynamics. Coastal lagoons along the California coast are already sensitive environments, subject to seasonal sandbar closure, variable salinity, and increasing fire pressure in their upstream catchments. A sudden influx of labile pyrogenic carbon could ripple through these systems in ways that measurements of black carbon alone would never reveal.

The broader implication is that wildfire’s impact on the global carbon cycle may be underestimated. Wildfire frequency and severity are increasing across the western United States and many other regions of the world, and each fire leaves behind a spectrum of organic matter ranging from the environmentally persistent to the rapidly degradable. If carbon accounting and biogeochemical models focus on the recalcitrant end of that spectrum, they will capture the long-term storage story but miss the fast-cycling one. The authors argue that expanding the focus beyond recalcitrant pyrogenic carbon to include labile compounds is essential to capture the full impact of terrestrial fires on carbon dynamics across the land-ocean aquatic continuum, the connected chain of rivers, lakes, estuaries, and coastal waters through which carbon travels from continents to the sea.

Methodologically, the study demonstrates the value of pairing molecular tracers with hydrologic context. By measuring levoglucosan and black carbon side by side across multiple burned watersheds and multiple flow conditions, the team could disentangle the effects of supply exhaustion from the effects of dilution and transport. The independence of the levoglucosan decline from discharge is a particularly telling signature: it points to a finite, depletable pool of soluble combustion products rather than a continuous source regulated by water flux. Such distinctions are exactly what models of post-fire carbon export need in order to predict how landscapes will behave as fire regimes intensify.

The research, supported by the National Science Foundation and the U.S. Department of Energy’s Office of Science, and conducted as part of Barton’s doctoral dissertation, arrives at a moment when fire science is racing to keep pace with a changing fire regime. As more watersheds burn and more rain falls on fresh ash, the first flush of labile carbon into rivers and coasts may prove to be one of the most dynamic and underappreciated consequences of wildfire. Recognizing that not all fire-derived carbon behaves alike, and that the most fragile molecules may carry the biggest early signal, is a crucial step toward understanding what burning landscapes mean for the carbon cycle from mountaintop to ocean.

Subject of Research: Post-fire mobilization of levoglucosan and black carbon in coastal California watersheds

Article Title: Early post-fire mobilization of levoglucosan distinctly contrasts black carbon dynamics in small coastal mountain watersheds

Article References: Barton, R., Wagner, S., Richardson, C. M., Collins, A., Zimmer, M. A., Montalvo, M., & Myers-Pigg, A. N. (2026). Early post-fire mobilization of levoglucosan distinctly contrasts black carbon dynamics in small coastal mountain watersheds. Biogeochemistry. https://doi.org/10.1007/s10533-026-01367-3

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01367-3

Keywords: levoglucosan, black carbon, pyrogenic carbon, wildfire, watersheds, biogeochemistry, carbon cycle, anhydrosugar, coastal lagoons, organic carbon, CZU Lightning Complex Fire, carbon export

The research, led by Lukas Fliri of Aalto University and BOKU University together with colleagues at Aalto and VTT Technical Research Centre of Finland, focused on fibers spun with the Ioncell technology, a process that dissolves pre-hydrolyzed kraft birch pulp in a superbase-derived ionic liquid and dry-jet wet spins it into filaments. Unlike the native cellulose I allomorph found in wood and cotton, these regenerated fibers crystallize in the cellulose II form, the same allomorph that matters for carbon fiber production. Yet, as the authors point out, nearly all of the fundamental mechanistic data on cellulose pyrolysis has been gathered on cellulose I, while studies on cellulose II have mostly reported end-material properties without the underlying physics and chemistry. That gap made it difficult for the team to compare their own carbonization experiments with the literature, so they decided to build the reference dataset themselves.

The experimental design deliberately echoes a classic: the influential 1960s work by Tang and Bacon on the carbonization of Fortisan cellulose II fibers. Fliri and colleagues heated Ioncell fibers from room temperature up to 1100 degrees Celsius under helium in a simultaneous thermal analyzer coupled to a mass spectrometer, using heating rates spanning 2.5 to 40 kelvin per minute. Tiny samples of around 3.5 milligrams minimized heat and mass transfer distortions, and the evolved gases, water, carbon monoxide and carbon dioxide, were quantified through a calibration procedure based on calcium oxalate monohydrate. In parallel, batch carbonizations were carried out in a tubular furnace under nitrogen at nine target temperatures between 300 and 1100 degrees Celsius, with no thermostabilization pretreatment and no tension applied to the fibers, so that the intrinsic behavior of the material could be observed without process tricks that boost yield.

The thermal analysis revealed the familiar fingerprint of cellulose pyrolysis: a violent mass loss concentrated in a window of only 70 to 100 degrees, with onset temperatures between 288 and 322 degrees Celsius depending on heating rate. At the peak of this event, cellulose chains depolymerize and expel levoglucosan, the sugar anhydride that dominates the volatile stream, alongside furans, dehydrated sugars, water and carbon oxides. Crucially, the slower the heating, the more char survived. Char yield at 1100 degrees Celsius fell log-linearly as the heating rate rose, and the combined yields of water and carbon dioxide tracked the char yield almost perfectly. This is exactly what the two-competing-pathway model of cellulose pyrolysis predicts: long residence times below 300 degrees favor dehydration reactions that lock carbon into char precursors, while rapid heating funnels the material toward volatilization as levoglucosan instead.

The kinetic analysis added quantitative teeth to this picture. Using Friedman’s isoconversional method, the team found that the apparent activation energy of Ioncell fiber pyrolysis sits in a narrow band of roughly 190 to 220 kilojoules per mole up to about 80 percent conversion, consistent with a single dominant step: the rupture of the beta-O-4 glycosidic bond that links the glucose units of cellulose. Beyond that conversion level, the activation energy climbs sharply, signaling a shift from depolymerization to reactions within the solid carbonaceous residue. A first-order kinetic model, fitted with activation energies of 227 to 231 kilojoules per mole and pre-exponential factors matching literature values for cellulose I, described the mass loss curves well, but the authors caution that this is a phenomenological description of the main mass loss step, not proof of a simple one-step mechanism, and the parameters should not be extrapolated to isothermal treatments or temperatures above about 600 degrees.

One of the more surprising results concerns heat. The apparent heat of pyrolysis, measured by integrating the differential scanning calorimetry signal, came out between roughly 180 and 250 kilojoules per kilogram across all heating rates, in close agreement with decades of measurements on microcrystalline cellulose and filter paper. When the team ran control experiments on pure levoglucosan, the heat of its thermal evaporation matched the apparent heat of cellulose pyrolysis almost exactly. The implication is striking: the endothermic demand of cellulose pyrolysis is largely just the energy needed to evaporate levoglucosan. From a process engineering standpoint, this reaction heat is small compared with the roughly 650 kilojoules per kilogram of sensible heat required simply to bring the fibers up to their degradation temperature, a fact that matters for anyone designing industrial carbonization furnaces.

The batch carbonization experiments then traced how the fibers’ properties evolve with the final heat treatment temperature. Yields dropped steeply between 300 and 500 degrees and then plateaued between 6.8 and 9.4 percent by weight. Elemental analysis showed hydrogen and oxygen draining away as carbon accumulated, and a van Krevelen plot revealed two distinct linear regimes: below about 400 degrees the hydrogen-to-carbon and oxygen-to-carbon ratios follow a slope of roughly two hydrogens per oxygen, the signature of dehydration, while above that temperature decarboxylation and decarbonylation take over. Notably, oxygen loss largely stops above 600 degrees, and X-ray photoelectron spectroscopy confirmed that thermally stable oxygen functionalities remain embedded in the carbon even at 1000 degrees. These trapped oxygen atoms, the authors argue, act like reticulation agents, promoting five- and seven-membered rings that curve and distort the growing aromatic network and prevent the formation of a graphite-like structure, a key reason cellulose-derived carbons are non-graphitizing.

Morphology told a parallel story. The round cross-section of the Ioncell filaments survived carbonization intact, but the diameter shrank by roughly 60 percent by the time the fibers reached 600 degrees, after which further shrinkage was minimal. Fiber diameter correlated linearly with carbon content and log-linearly with carbonization yield, and the team interprets the shrinkage as the surface degradation of oriented cellulose microfibrils followed by their coalescence into denser cross-linked furanic and polyaromatic structures. Raman spectroscopy completed the picture: a polyaromatic network was already detectable at 400 degrees, and the ratio of the disorder-induced D band to the graphitic G band jumped from 0.59 to 0.93 between 600 and 700 degrees, then climbed more slowly toward 1100 degrees. The D-to-G intensity ratio correlated linearly with the hydrogen-to-carbon ratio above 600 degrees, indicating that large aromatic domains grow through dehydrogenation and radical condensation of smaller ring systems. Estimated in-plane aromatic cluster sizes grew from about 1.0 to 1.4 nanometers between 600 and 800 degrees and then stalled.

Perhaps the most consequential conclusion is what the study did not find. Older carbon fiber literature, still frequently cited, holds that the cellulose II crystal structure follows a special carbonization mechanism tied to the recondensation of four-carbon intermediates. The new data show no evidence of such a peculiarity: the pyrolysis behavior of Ioncell fibers matched the classic two-pathway models, and the high-temperature evolution agreed with general models for non-graphitizable carbons. The authors also observed an unexplained systematic mass loss above 800 degrees, but only at the slowest heating rate of 2.5 kelvin per minute, a phenomenon not previously reported that they flag for further confirmation. By identifying the temperature windows where dehydration, volatilization, dehydrogenation and aromatization each dominate, the dataset gives carbon fiber researchers a practical map for optimizing thermostabilization and carbonization schedules, and a common reference point for comparing cellulose II precursors of every kind. For a field trying to make carbon fiber cheap, renewable and strong enough for the mass market, that map may prove as valuable as the fibers themselves.

Subject of Research: Pyrolysis dynamics and physicochemical property development during the carbonization of Ioncell regenerated cellulose II fibers for carbon fiber production

Article Title: Tracking carbonization in Ioncell cellulose II fibers: pyrolysis dynamics and development of the physicochemical properties

Article References: Fliri, L., Yildirim, Z., Meinander, K., Guizani, C., & Hummel, M. (2026). Tracking carbonization in Ioncell cellulose II fibers: pyrolysis dynamics and development of the physicochemical properties. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13716-6

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13716-6

Keywords: carbon fibers, cellulose II, Ioncell, pyrolysis, carbonization, levoglucosan, char yield, Raman spectroscopy, thermogravimetric analysis, biobased materials, turbostratic carbon, kinetic modeling